The present invention relates to a transmitter for a portable radio communication apparatus, and more particularly to a modulator for a direct conversion transmitter.
A general trend in portable communication apparatus is the reduction in volume, weight and power consumption of such apparatus. This has led to efforts towards reducing the number of elements and devices necessary to perform the functions associated with portable communication devices. In particular, the radio frequency transmit strip of portable communication apparatus, which typically comprises a number of up-converting stages, is an area in which a reduction in the number of elements and devices would be beneficial.
One approach to reduce the number of stages in the radio frequency transmit strip is to convert a baseband signal (comprising the information to be transmitted) to a radio frequency carrier signal in a single step. This is termed direct conversion or direct modulation. To carry out direct conversion, a local oscillator signal (LO) having the same frequency as the required radio frequency carrier signal is mixed with the baseband signal in a suitable non-linear device such as a mixer diode. The output of the mixer contains the sum and difference of the LO and the baseband signal. In this way, the LO signal is modulated by the baseband signal.
Typically, the baseband signal comprises ‘I’ and ‘Q’ components and accordingly two such modulators with their outputs summed together are required. These are fed with two LO signals, the ‘Q’ component LO having a 90 degree phase shift with respect to the ‘I’ component.
Therefore, one of the issues with direct conversion transmitter design is that of generating the 90 phase shift for the ‘Q’ LO signal. At present, there are two commonly used methods: one is a passive phase shift network using reactive elements, the other is an active divide-by-two circuit. The passive phase shift network approach involves the design of a frequency selective ‘All Pass’ filter network to provide the phase difference between the ‘I’ and ‘Q’ local oscillators but has the disadvantage that it is inherently limited to substantially narrow band applications and also that it can be difficult to integrate such networks onto an IC. The divide-by-two approach uses a pair of dividers one clocked off the rising and one off the falling edge of an LO at double the wanted operating frequency. The active divide-by-two circuit has the disadvantage that it requires high current to operate and requires an LO of double the frequency of operation. For example, if the wanted operating frequency were say 1.9 GHz then an LO of 3.8 GHz would be required to drive this circuit. Hence, a synthesizer and divide-by-two operating at 3.8 GHz would consume a great deal of current. Since the trend in portable communication devices is towards compactness there is less space for battery packs, and thus less battery capacity is available and so low current operation becomes increasingly important to achieve acceptable talk and standby times.
A separate issue, but one that is also important in the design of a direct conversion transmitter, is that of the gain control of the transmitter. In certain radio telephone systems such as WCDMA the transmitter is required to vary its output power over a very wide range e.g. 70 dB for WCDMA. Additionally, a direct conversion transmitter must obtain all of its control range at radio frequencies. Thus, since the LO is operating at the wanted radio frequency, problems of LO leakage and shielding can become very significant. Conventionally, with one or more IFs, variable gain amplifiers and/or attenuators are distributed between the RF and intermediate frequency (baseband) and used to vary the transmitter power. These approaches however have not satisfactorily provided the wide range of power control required.
A further concern with direct conversion transmitters is that the LO signal cannot be provided directly from a synthesiser locked VCO. There are two main reasons why. Firstly, if the radiotelephone has an internal antenna there is a very great risk that the transmitter will radiate back into the synthesiser locked VCO and cause it to go out of lock or generate spurious signals. Secondly, there will be insufficient isolation between antenna impedance (which will vary a great deal as the user moves around) and the synthesiser locked VCO. This will cause the synthesiser locked VCO to either go out of lock or generate spurious signals. One method of solving this problem is to create the LO signal by mixing together two synthesiser locked VCO signals and then filtering the LO to remove any unwanted mixing products. This however, increases component count and current consumption.